1. Field of the Invention
The present invention relates to lithography, and more particularly, to lithography gas gauges.
2. Background of Invention
Many automated manufacturing processes require the sensing of the distance between a manufacturing tool and the product or material surface being worked. In some situations, such as semiconductor lithography, the distance must be measured with accuracy approaching a nanometer.
The challenges associated with creating a proximity sensor of such accuracy are significant, particularly in the context of photolithography systems. In the photolithography context, in addition to being non-intrusive and having the ability to precisely detect very small distances, the proximity sensor can not introduce contaminants or come in contact with the work surface, typically a semiconductor wafer. Occurrence of either situation may significantly degrade or ruin the semiconductor quality.
Different types of proximity sensors are available to measure very small distances. Examples of proximity sensors include capacitance and optical gauges. These proximity sensors have serious shortcomings when used in photolithography systems because physical properties of materials deposited on wafers may impact the precision of these devices. For example, capacitance gauges, being dependent on the concentration of electric charges, can yield spurious proximity readings in locations where one type of material (e.g., metal) is concentrated. Another class of problems occurs when exotic wafers made of non-conductive and/or photosensitive materials, such as Gallium Arsenide (GaAs) and Indium Phosphide (InP), are used. In these cases, capacitance and optical gauges may provide spurious results.
U.S. Pat. No. 4,953,388, entitled Air Gauge Sensor, issued Sep. 4, 1990 to Andrew Barada (“'388 patent”), and U.S. Pat. No. 4,550,592, entitled Pneumatic Gauging Circuit, issued Nov. 5, 1985 to Michel Deschape (“'592 patent”), disclose an alternative approach to proximity sensing that uses an air gauge sensor. U.S. Pat. Nos. 4,953,388 and 4,550,592 are incorporated herein in their entireties. Furthermore, principles of pneumatic gauging are discussed in Burrows, V. R., The Principles and Applications of Pneumatic Gauging, FWP Journal, October 1976, pp. 31–42, which is incorporated herein in its entirety. An air gauge sensor is not vulnerable to concentrations of electric charges or electrical, optical and other physical properties of a wafer surface. Current semiconductor manufacturing, however, requires that proximity is gauged with high precision on the order of nanometers. Earlier versions of air gauge sensors, however, often do not meet today's lithography requirements for precision.
Co-pending, commonly owned U.S. patent application Ser. No. 10/322,768, entitled High Resolution Gas Gauge Proximity Sensor, filed Dec. 19, 2002 by Gajdeczko et al., (“'768 patent application”) describes a high precision gas gauge proximity sensor that overcomes some of the precision limitations of earlier air gauge proximity sensors. The 768 patent application, which is incorporated herein in its entirety, describes a gas gauge proximity sensor that provides a high degree of accuracy. Similarly, co-pending, commonly owned U.S. patent application Ser. No. 10/683,271, entitled Liquid Flow Proximity Sensor for Use in Immersion Lithography, filed Oct. 14, 2003, by Violette, Kevin, (“'271 patent application”) describe a high precision immersion lithography proximity sensor that provides a high degree of precision in an immersion lithography application.
While the sensors disclosed in the '768 and '271 patent applications provide a high degree of precision, the precision can be impacted by cross flows of gas or liquid that intersect the stream of gas or liquid that is being emitted from a measurement channel of the proximity sensor. Specifically, purging gases, for example, can exhibit local cross winds with velocities of the order of a few meters per second. Cross-winds or cross-flows will cause gauge instability and drift, introducing non-calibratable errors within proximity sensors.
What is needed is an apparatus to neutralize these cross-flows to improve the accuracy of proximity sensors.